Method and apparatus for providing orthogonal spot beams, sectors, and picocells

ABSTRACT

A method and apparatus for providing orthogonal spot beams ( 14   a,    14   b ), sectors ( 16   a,    16   b ), and picocells ( 18 ), by using orthogonal auxiliary pilots and different Walsh traffic channels in adjacent areas. According to the IS-95 standard, the pilot signal is covered with the 64-chip Walsh sequence zero. Designating the 64-chip all zeros Walsh sequence as P and the 64-chip all one sequence as M, additional pilot signals are provided in the present invention by concatenating the P and the M sequences. Thus, for two pilot signals, pilot Walsh sequences of PP and PM can be used. For four pilot signals, pilot Walsh sequences of PPPP, PMPM, PPMM, and PMMP can be used. In general, the required number of pilot Walsh sequences can be generated by substituting each bit in an K-bit Walsh sequence with the 64-chip all zeros P or all ones M sequence, depending on the value of that bit.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation of patentapplication Ser. No. 09/852,389, entitled “METHOD AND APPARATUS FORPROVIDING ORTHOGONAL SPOT BEAMS, SECTORS, AND PICOCELLS” filed May 9,2001, and will issue as U.S. Pat. No. 7,031,282 on Apr. 18, 2006, whichis a Continuation of patent application Ser. No. 08/925,521, entitled“METHOD AND APPARATUS FOR PROVIDING ORTHOGONAL SPOT BEAMS, SECTORS, ANDPICOCELLS” filed Sep. 8, 1997, and issued as U.S. Pat. No. 6,285,655,both assigned to the assignee hereof and hereby expressly incorporatedby reference herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications. More particularly, thepresent invention relates to a method and apparatus for providingorthogonal spot beams, sectors, and picocells.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques isone of several techniques for facilitating communications in which alarge number of system users are present. Although other techniques suchas time division multiple access (TDMA), frequency division multipleaccess (CDMA), and AM modulation schemes such as amplitude compandedsingle sideband (ACSSB) are known, CDMA has significant advantages overthese other techniques. The use of CDMA techniques in a multiple accesscommunication system is disclosed in U.S. Pat. No. 4,901,307, entitled“SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE ORTERRESTRIAL REPEATERS,” and assigned to the assignee of the presentinvention and incorporated by reference herein. The use of CDMAtechniques in a multiple access communication system is furtherdisclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FORGENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”,assigned to the assignee of the present invention and incorporated byreference herein. The CDMA system can be designed to conform to the“TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System”, hereinafterreferred to as the IS-95 standard.

The CDMA system is a spread spectrum communication system. The benefitsof spread spectrum communication are well known in the art and can beappreciated by reference to the above cited references. CDMA, by itsinherent nature of being a wideband signal, offers a form of frequencydiversity by spreading the signal energy over a wide bandwidth.Therefore, frequency selective fading affects only a small part of theCDMA signal bandwidth. Space or path diversity is obtained by providingmultiple signal paths through simultaneous links to a mobile user orremote station through two or more base stations. Furthermore, pathdiversity may be obtained by exploiting the multipath environmentthrough spread spectrum processing by allowing signals arriving withdifferent propagation delays to be received and processed separately.Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF INCOMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No.5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONESYSTEM,” both assigned to the assignee of the present invention andincorporated by reference herein.

In a CDMA system, the forward link refers to a transmission from a basestation to a remote station. In the exemplary CDMA communication systemwhich conforms to the IS-95 standard, forward link data and voicetransmissions occur over orthogonal code channels. In accordance withthe IS-95 standard, each orthogonal code channel is covered with aunique Walsh sequence which is 64 chips in duration. The orthogonalityminimizes the interference between the code channels and improvesperformance.

CDMA systems offer higher system capacity, as measured by the number ofsupportable users, through several design features. First, the transmitfrequency of adjacent cells can be reused. Second, increased capacitycan be achieved by using more directive antennas for the transmission tosome areas or to some remote stations. In the CDMA system, the coveragearea (or cell) can be divided into several (e.g., three) sectors usingdirective antennas. The method and apparatus for providing sectors in aCDMA communication system is described in U.S. Pat. No. 5,621,752,entitled “ADAPTIVE SECTORIZATION IN A SPREAD SPECTRUM COMMUNICATIONSYSTEM”, assigned to the assignee of the present invention andincorporated by reference herein. Each sector or cell can be furtherdivided into more directive spot beams. Alternatively, spot beams can beassigned to selected remote stations or a set of remote stations withina sector or a cell. A picocell is a localized coverage area within asector or a cell. The picocell can be embedded within a sector or a cellto improve capacity and provide additional services.

In the exemplary CDMA system, the forward link transmissions indifferent sectors typically use different short PN spreading sequences(or different offsets of a common set of short PN spreading sequences).Thus, when a remote station is in overlapping sector coverage areas anddemodulating the signal from one sector, the signals from other sectorsare spread and appear as wideband interference. However, the signalsfrom other sectors or cells are not orthogonal to each other. Thenon-orthogonal interference from adjacent sectors or cells can degradethe performance of the communication system.

In an IS-95 CDMA communication system, a pilot channel is transmitted onthe forward link to assist the remote station perform coherentdemodulation of the received signal. Coherent demodulation results inimproved performance. For each beam, a pilot channel is utilized. Inaccordance with the IS-95 standard, the pilot channel is covered withWalsh sequence zero.

A number of challenges arise when attempting to increase the capacity ofthe CDMA system. First, the Walsh sequences available for covering thecode channels is defined by the IS-95 standard and limited to 64.Second, a method is desired to allow the remote stations to distinguishthe different beams, sectors, or picocells in CDMA systems with minimalsignal processing. And third, maintaining conformance to the IS-95standard is a desirable condition. The present invention addresses thesechallenges.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus forproviding orthogonal spot beams, sectors, and picocells. Thetransmissions can be made orthogonal by using orthogonal auxiliarypilots and different Walsh traffic channels in adjacent areas. Inaccordance with the IS-95 standard, the pilot signal is covered with the64-chip all zeros Walsh sequence. In the exemplary embodiment, the64-chip all zeros Walsh sequence is designated as P and the 64-chip allones sequence is designated as M. In the present invention, additionalpilot signals can be provided by concatenating the 64-chip all zeros Pand all ones M sequences. For two pilot signals, pilot Walsh sequencesof PP and PM can be used. For four pilot signals, pilot Walsh sequencesof PPPP, PMPM. PPMM, and PMMP can be used. The present invention can beextended such that K pilot Walsh sequences can be generated bysubstituting each bit in an K-bit Walsh sequence with the 64-chip allzeros P or all ones M sequence depending on the value of that bit. Usingthis method, K pilot Walsh sequences can be generated from the basic allzeros P and all ones M sequences, where K is a number which is a powerof two.

It is an object of the present invention to provide orthogonal spotbeams, sectors, and picocells. In the exemplary embodiment, the trafficchannels in a transmission area are covered with Walsh sequences whichare orthogonal to those of adjacent areas. In addition, the pilot foreach transmission area is covered with pilot Walsh sequence which isderived from Walsh sequence zero. Orthogonal traffic channels and pilotsminimize interference and improve capacity.

It is another object of the present invention to provide additionalorthogonal pilot channels without reducing the number of orthogonalWalsh channels available for traffic and control channels. In accordancewith the IS-95 standard, 64 Walsh sequences are available for covering64 code channels. Walsh sequence zero is reserved for the pilot channeland the remaining 63 Walsh sequences can be used for other codechannels, such as traffic channels and control channels. In the presentinvention, the additional pilot signals are generated using concatenatedcombinations of the all zeros and all ones sequences. All pilot signalsare orthogonal to each other and to the remaining Walsh sequences. Theremaining 63 Walsh sequences are still available for system use.

It is yet another object of the present invention to provide anefficient mechanism to search and distinguish the pilot signals ofdifferent beams, sectors, and picocells in CDMA systems. In theexemplary embodiment, the pilot signals are spread using the same shortspreading sequence. The remote station is able to despread all pilotsignals using the same short despreading sequence. For each 64-chipinterval, the length of the basic Walsh sequence, the despread signal isdecovered with Walsh sequence zero to provide I and Q pilot values. Foreach pilot signal hypothesis, the I and Q pilot values obtained from thepresent and previous 64-chip intervals are combined in accordance withthe hypothesis and the decovered pilot is compared against predeterminedthresholds. Since all pilot signal hypotheses can be computed from thecommon set of I and Q pilot values, the signal processing to receive anddistinguish the pilot signals from different beams, sectors, andpicocells can be easily performed.

It is another object of the present invention to provide an efficientmechanism to add and drop beams, sectors, and picocells from the activeand/or candidate sets of the remote station. In the exemplaryembodiment, each remote station maintains an active set comprising thelist of beams, sectors, and picocells with which the remote station isin active communication. In the exemplary embodiment, each remotestation also maintains a candidate set comprising the list of beams,sectors, and picocells from which the energy of the received pilotsignals exceed a predetermined threshold. The energy of the receivedpilot signals can be computed from the decovered pilot. If the energy isabove an add threshold, the spot beam, sector, or picocell correspondingto this pilot signal can be added to the active/candidate set of theremote station. Alternatively, if the energy is below a drop threshold,the spot beam, sector, or picocell corresponding to this pilot signalcan be removed from the active/candidate set.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1A is a diagram of an exemplary CDMA cell comprising a wider beamand a plurality of spot beams;

FIG. 1B is a diagram of an exemplary CDMA cell comprising three sectorsand a picocell;

FIG. 2 is a block diagram of an exemplary forward link transmission andreceiving subsystem of the present invention;

FIG. 3 is a block diagram of an exemplary channel element within thebase station; and

FIG. 4 is block diagram of an exemplary demodulator within the remotestation.

FIG. 5 is a block diagram of EB/Nt with respect to the Distance frompicocell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method and apparatus for providing orthogonalspot beams, sectors, and picocells. In accordance with the IS-95standard, the forward link comprises 64 orthogonal code channels whichare generated by covering each code channel with one of 64 unique Walshsequences. In accordance with the IS-95 standard, Walsh sequence zero isreserved for the pilot signal. To increase capacity, the forward linktransmission can comprise multiple transmissions. Each transmission canbe directed to a particular area by the use of directive antennas. Forexample, a transmission can be directed at the entire area surroundingthe base station (e.g., an omni-directional transmission), a sector of acell, or a localized area within a sector or a cell using spot beams orpicocells. Spot beams provide antenna gain, minimize interference, andincrease capacity. In this specification, a particularized transmissioncomprises a transmission covering a cell, a sector, or a picocell and adirective transmission using a wider beam, a spot beam, or otherdirective beams.

For coherent demodulation, the phase of a pilot signal is used todemodulate the received signal. In the exemplary embodiment, one pilotsignal is transmitted with each particularized transmission. In theexemplary embodiment, to minimize interference to adjacent area,transmissions are provided through orthogonal channels. However, thenumber of Walsh sequences available for covering code channels is fixedfor an IS-95 system. A method and apparatus is required to provideadditional orthogonal pilot channels, as required by beams, orthogonalsectors, and picocells without utilizing pre-exiting Walsh sequencessince that would reduce the number of available Walsh sequences whichcan be used to cover traffic and control channels. In addition,maintaining capability with the IS-95 standard is an importantconsideration.

In accordance with the IS-95 standard, each Walsh sequence is 64 chipsin duration. Furthermore, the Walsh sequence reserved for the pilotchannel is the all zeros sequence. In the present invention, theadditional orthogonal pilot channels are provided by concatenating theall ones and all zeros sequences. The all ones and all zeros sequencesare orthogonal to all other Walsh sequences. The additional longer pilotWalsh sequences provided by the present invention are orthogonal to eachother and the other 64-chip non-pilot Walsh sequences.

In the exemplary embodiment, the 64-chip all zeros Walsh sequence isdesignated as P and the 64-chip all ones sequence is designated as M. Inthe present invention, additional orthogonal pilot Walsh sequences canbe provided by concatenating sequences of P and M. For example, twopilot channels can be provided by using 128-chip pilot Walsh sequencesobtained with a 2-bit type of Walsh code mapping of P and M. Thus, pilotWalsh sequences of PP and PM can be used. The PM pilot Walsh sequencecomprises a 64-bit all zeros sequence immediately followed by a 64-bitall ones sequence. Similarly, four pilot channels can be provided byusing 256-chip pilot Walsh sequences obtained with a 4-bit type of Walshcode mapping of P and M. Thus, pilot Walsh sequences of PPPP, PMPM,PPMM, and PMMP can be used. The PMPM pilot Walsh sequence comprises a64-bit all zeros sequence immediately followed by a 64-bit all onessequence immediately followed by a 64-bit all zeros sequence andimmediately followed by a 64-bit all ones sequence. The concept can befurther extended to provide K pilot channels using correspondinglylonger (e.g., 64·K) pilot Walsh sequences. In the exemplary embodiment,the all zeros sequences (e.g., PP and PPPP) are reserved for the“original” pilot channel (e.g., for the wider beam or theomni-directional transmission) to maintain compliance with the IS-95standard.

Many benefits are provided by the pilot channels generated in accordancewith the present invention. First, the number of Walsh sequencesavailable for other code channels is not affected (or reduced) by theadditional pilot channels. Second, in the exemplary embodiment, the sameshort PN offset is utilized for all pilot channels so that searching forpilot signals of spot beams, sectors, and picocells is simplified.Third, the addition or removal of beams, sectors, or picocells to orfrom the active and/or candidate sets of a remote station is simplified.And finally, the interference of the pilot channel to adjacent areas isminimal since the pilot channels are orthogonal. The interference oftraffic channels is also minimal if the traffic channels in the adjacentareas use different Walsh channels. These benefits are described below.

Referring to the figures, FIG. 1A is a diagram of an exemplary CDMAcell. The forward link transmission from base station 4 to remotestation 6 can comprise wider beam (or omni-directional beam) 12 and spotbeams 14 a and 14 b. As shown in FIG. 1A, spot beams 14 can be directedat different geographic coverage areas and can have different sizes.Spot beams 14 can be used to increase capacity and improve performance.Base station 4 can transmit to zero or more remote stations 6 within anybeam. For example, in FIG. 1A, base station 4 transmits to remotestation 6 a using wider beam 12, to remote stations 6 b and 6 c usingspot beam 14 a, and to remote station 6 d using spot beam 14 b.

FIG. 1B is a diagram of another exemplary CDMA cell. The CDMA cell canbe partitioned into sectors 16 in the manner described in theaforementioned U.S. Pat. No. 5,621,752. Picocell 18 is a localizedtransmission which is embedded within sector 16 a. As shown in FIG. 1B,base station 4 can transmit to zero or more remote stations 6 within anysector 16 or picocell 18. For example, in FIG. 1B, base station 4transmits to remote station 6 e in sector 16 a, to remote stations 6 fand 6 g in sector 16 b, to remote station 6 h in sector 16 c, and toremote station 6 i in sector 16 a and picocell 18.

A block diagram of an exemplary forward link transmission and receivinghardware is shown in FIG. 2. Within base station 4, data source 110contains the data to be transmitted to remote station 6. The data isprovided to channel element 112 which partitions the data, CRC encodesthe data, and inserts code tail bits as required by the system. Channelelement 112 then convolutionally encodes the data, CRC parity bits, andcode tail bits, interleaves the encoded data, scrambles the interleaveddata with the user long PN sequence, and covers the scrambled data witha Walsh sequence. The traffic channel and pilot channel datacorresponding to each particularized transmission (e.g., each spot beam,sector, or picocell) is combined and provided to a modulator andtransmitter (MOD AND TMTR) 114 (only one is shown in FIG. 2 forsimplicity). Each modulator and transmitter 114 spreads the covered datawith the short PN_(I) and PN_(Q) sequences. The spread data is thenmodulated with the in-phase and quadrature sinusoids, and modulatedsignal is filtered, upconverted, and amplified. The forward link signalis transmitted on forward link 120 through antenna 116.

At remote station 6, the forward link signal is received by antenna 132and provided to receiver (RCVR) 134. Receiver 134 filters, amplifies,downconverts, quadrature demodulates, and quantizes the signal. Thedigitized data is provided to demodulator (DEMOD) 136 which despreadsthe data with the short PN_(I) and PN_(Q) sequences, decovers thedespread data with the Walsh sequence, and derotates the decovered datawith the recovered pilot signal. The derotated data from differentcorrelators within demodulator 136 are combined and descrambled with theuser long PN sequence. The descrambled (or demodulated) data is providedto decoder 138 which performs the inverse of the encoding performedwithin channel element 112. The decoded data is provided to data sink140.

A block diagram of an exemplary channel element 112 is shown in FIG. 3.In the exemplary embodiment, channel element 112 comprises at least onetraffic channel (or code channel) 212 and at least one pilot channel232. Within each traffic channel 212, CRC encoder 214 receives thetraffic data, performs CRC encoding, and can insert a set of code tailbits in accordance with the IS-95 standard. The CRC encoded data isprovided to convolutional encoder 216 which encodes the data with aconvolutional code. In the exemplary embodiment, the convolutional codeis specified by the IS-95 standard. The encoded data is provided tointerleaver 218 which reorders the code symbols within the encoded data.In the exemplary embodiment, interleaver 218 is a block interleaverwhich reorders the code symbols within blocks of 20 msec of encodeddata. The interleaved data is provided to multiplier 220 which scramblesthe data with the user long PN sequence. The scrambled data is providedto multiplier 222 which covers the data with the Walsh sequence assignedto this traffic channel 212. The covered data is provided to gainelement 224 which scales the data such that the requiredenergy-per-bit-to-noise E_(b)/I₀ ratio is maintained at remote station 6while minimizing transmit power. The scaled data is provided to switch230 which directs the data from traffic channel 212 to the proper summer240. Summers 240 sum the signals from all traffic channels 212 and pilotchannel 232 designated for a particularized transmission. The resultantsignal from each summer 240 is provided modulator and transmitter 114which functions in the manner described above.

Channel element 112 comprises at least one pilot channel 232. The numberof pilot channels 232 required is dependent on the system requirements.For each pilot channel 232, the pilot data is provided to multiplier 234which covers the data with a pilot Walsh sequence. In the exemplaryembodiment, the pilot data for all pilot channels 232 are identical andcomprises the all ones sequence. The covered pilot data is provided togain element 236 which scales the pilot data with a scaling factor tomaintain the required pilot signal level. The scaled pilot data isprovided to switch 230 which directs the data from pilot channel 232 tothe proper summer 240.

The hardware as described above is one of many embodiments which supportmultiple particularized transmissions from base station 4. Otherhardware architectures can also be designed to perform the functionsdescribed herein. These various architectures are within the scope ofthe present invention.

In the exemplary embodiment, the Walsh sequence provided to each trafficchannel 212 is a 64-bit Walsh sequence as defined by the IS-95 standard.In the exemplary embodiment, Walsh sequence zero is reserved for thepilot channels. In the exemplary embodiment, the pilot Walsh sequenceprovided to each pilot channel 232 is generated from concatenation ofthe 64-bit all zeros and all ones sequences. The number of pilotchannels required determines the minimum length of the pilot Walshsequences. In the exemplary embodiment, for two pilot channels, thelength of the pilot Walsh sequence is 128 bits and for four pilotchannels, the length of the pilot Walsh sequence is 256 bits. The lengthof the pilot Walsh sequence can be generalized as 64·K, where K is thenumber of pilot channels required by base station 4 and is a power oftwo. For four pilot channels, the pilot Walsh sequences can be PPPP,PMPM, PPMM, and PMMP, where P and M are defined above.

In the exemplary embodiment, a pilot signal is transmitted with eachparticularized transmission. Referring to FIG. 1A, spot beams 14 a and14 b require transmission of two addition pilot signals. Additionaltransmit power is required for the additional pilot signals. However,because of higher antenna gain associated with the directivity of spotbeams 14, the required transmit power for the pilot signal and forwardlink signal for each spot beam 14 is reduced by the antenna gain. Thus,higher capacity can be achieved even in the presence of additionaltransmissions of the pilot signals. In fact, in the present invention,the transmit power of the forward traffic channels and pilot channel canbe adjusted (possibly dynamically) in accordance with the directivity ofthe particularized transmission (e.g., the antenna gain of the spotbeam).

A block diagram of an exemplary demodulator within remote station 6 isshown in FIG. 4. The forward link signal is received by antenna 132 andprovided to receiver 134 which processes the signal in the mannerdescribed above. The digitized I and Q data is provided to demodulator136. Within demodulator 136, the data is provided to at least onecorrelator 310. Each correlator 310 processes a different multipathcomponent of the received signal. Within correlator 310, the data isprovided to complex conjugate multiplier 320 which multiplies the I andQ data with the short PN_(I) and PN_(Q) sequences to obtain the despreadI and Q data. The complex conjugate multiplier removes the spreadingperformed by the complex multiplier within modulator and transmitter114.

The despread I and Q data is provided to multipliers 322 a and 322 b andpilot correlators 326 a and 326 b, respectively. Multipliers 322 a and322 b multiply the I and Q data with the Walsh sequence (Wx) assigned tothat correlator 310. The I and Q data from multipliers 322 a and 322 bis provided to accumulators (ACC) 324 a and 324 b, respectively. In theexemplary embodiment, accumulators 324 accumulate the data over the64-chip interval, the length of the Walsh sequence. The decovered I andQ data from accumulators 324 is provided to dot product circuit 328.Pilot correlators 326 a and 326 b decover the I and Q data with thepilot Walsh sequence (PWy) assigned to that correlator 310 and filterthe decovered pilot signal. The operation of pilot correlators 326 isdescribed below. The filtered pilot is provided to dot product circuit328. Dot product circuit 328 computes the dot product of the two vectors(the pilot and data) in a manner known in the art. An exemplaryembodiment of dot product circuit 328 is described in detail in U.S.Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT”,assigned to the assignee of the present invention and incorporated byreference herein. Dot product circuit 328 projects the vectorcorresponding to the decovered data onto the vector corresponding to thefiltered pilot, multiplies the amplitude of the vectors, and provides asigned scalar output to combiner 330. Combiner 330 combines the outputsfrom correlators 310 which have been assigned to demodulate the receivedsignal and routes the combined data to long PN despreader 332. Long PNdespreader 332 despreads the data with the long PN sequence and providesthe demodulated data to decoder 138.

The operation of pilot correlator 326 is described as follows. In theexemplary embodiment, the pilot signals from particularizedtransmissions are spread with the same short PN sequence but coveredwith different pilot Walsh sequences. For each sequence interval, whichis 64-chip in duration for the exemplary IS-95 Walsh sequence, the pilotsignals from the in-phase and quadrature channels are accumulated andstored as the I and Q pilot values, respectively. The I and Q pilotvalues for the current sequence interval are combined with the I and Qpilot values for previous sequence intervals in accordance with thepilot hypothesis being searched. As an example, assume that I₀ and Q₀are the pilot values accumulated for the current sequence interval, andI₁ and Q₁, I₂ and Q₂, and I₃ and Q₃ are the pilot values accumulated forthe immediately prior three sequence intervals. Then, for the PPPP pilothypothesis, the decovered pilot comprises I_(d,PPPP)=I₀+I₁+I₂+I₃ andQ_(d,PPPP)=Q₀+Q₁+Q₂+Q₃. Similarly, for the PMPM pilot hypothesis, thedecovered pilot comprises I_(d,PMPM)=I₀−I₁+I₂−I₃ andQ_(d,PMPM)=Q₀−Q₁+Q₂−Q₃. Thus, the decovered pilot for all pilothypotheses can be calculated from the one set of I and Q pilot values.The energy of the decovered pilot can be computed as E_(p)=I_(d) ²+Q_(d)².

Many benefits are provided by the pilot channels generated in accordancewith the present invention. First, the number of Walsh sequencesavailable for other code channels is not affected (or reduced) since 63are still available for the traffic channels and only Walsh sequencezero is used for the pilot channels. This is particularly important whencapacity, in terms of the number of remote stations supportable by basestation 4, is sought to be increased with minimal changes to the CDMAarchitecture as defined by the IS-95 standard.

Second, in the exemplary embodiment, the same short PN offset isutilized for all pilot channels so that searching for and distinguishingpilot signals from particularized transmissions are simplified. In theprior art sectored cell, the pilot signal of each sector is spread withshort PN sequences having different offsets. At remote station 6, asearch of the pilot signals requires despreading the received signalwith different short PN sequences, each having a different offsetcorresponding to that of the sector. In the exemplary embodiment, thepilot signals of particularized transmissions are spread with the sameshort PN sequences but covered with different pilot Walsh sequences.Thus, the pilot signal is only despread once and the decovered pilot fordifferent pilot hypotheses can be computed from the common set of I andQ pilot values as described above.

Third, the addition or removal of spot beams, sectors, and picocells toor from the active set and/or the candidate set of remote station 6 issimplified by the present invention. In the exemplary embodiment, remotestation 6 can treat the pilot signals covered with the pilot Walshsequence in a manner similar to those from other sectors and cells.Specifically, the set of active and candidate pilots can be maintainedby comparing the energy obtained by the searcher pilot correlator 326with a set of predetermined thresholds. If the energy E_(p) of the pilotsignal is above an add threshold, the particularized transmissioncorresponding to this pilot signal can be added to the active/candidateset of remote station 6. Alternatively, if the energy E_(p) of the pilotsignal is below a drop threshold, the particularized transmissioncorresponding to this pilot signal can be removed from theactive/candidate set. Similarly, handoff between particularizedtransmissions can be handled in a manner similar to that performed inIS-95 systems.

I. Auxiliary Pilots for Sectored Cells

The present invention can be utilized to provide improved performancefor sectored cells. In accordance with the IS-95 standard, each sectoredcell uses a different PN offset of a common PN sequence on the forwardlink. This architecture does not provide forward link signals which areorthogonal to each other and this can limit the performance of the link.For example, if remote station 6 is close to base station 4, the pathloss is small. This enables transmissions of high rate data over thelink. However, if remote station 6 is between two sectors, remotestation 6 receives a considerable amount of non-orthogonal signalinterference. This non-orthogonal signal interference, rather thanthermal noise, limits the maximum data rate that the link can support.If the sectors transmit signals that are orthogonal to each other, theother-sector signal interference is minimized and transmissions athigher data rates are possible with just thermal noise and some residualnon-orthogonal signal interference. With orthogonal signals, performancein the areas covered by more than one antenna is also improved by thediversity provided by the multiple paths.

The orthogonal signals are provided by using different orthogonalauxiliary pilots for the sectors, using different Walsh traffic channelsfor the traffic in adjacent sectors, and minimizing the time differencebetween the signals received from the adjacent sectors. This timedifference can be accomplished by using sector antennas that are inclose proximity to each other so that the path delay between theantennas is smaller than the chip period. The timing of the sectors canalso be adjusted to compensate for time differences.

II. Auxiliary Pilots for Picocells

The present invention can be used to provide additional pilots forpicocells. The picocell can comprise a localized coverage area which canbe used to provide additional services. The picocell can reside (or beembedded) within a macrocell and the macrocell can be a cell, a sector,or a beam. In one implementation, the picocell can be implemented usingdifferent transmission frequencies. However, this may not be feasible oreconomically practical. The present invention can be use to provideseparate pilots for picocells.

In the exemplary embodiment, a set of Walsh sequences which are not usedby the macrocell can be used by the picocell. In the exemplaryembodiment, the picocell aligns its transmit timing to that of themacrocell. This can be accomplished by one of many embodiments. In theexemplary embodiment, a receiver at the picocell receives the forwardlink signals from the picocell and the macrocell and adjusts the timingof the picocell so that it is aligned with that of the macrocell. Aftertime alignment of the transmissions of the picocell with those of themacrocell, the transmissions from the picocell can be made orthogonal tothose of the macrocell at the center of the picocell by using orthogonalauxiliary pilots and different Walsh traffic channels for the data inthe cells.

A diagram a picocell 18 embedded within a macrocell (or sector 16 a) isshown in FIG. 1B. Line 20 passes through the center of picocell 18. Adiagram of the energy-per-bit-to-total-interference-density ratio,E_(b)/N_(t), of a remote station 6 along line 20 is shown in FIG. 5. InFIG. 5, the E_(b)/N_(t) of a picocell which radiates in a manner whichis orthogonal to the macrocell and a picocell which does not radiateorthogonally to the macrocell are shown.

FIG. 5 shows that there is only a small degradation from the orthogonalpicocell to the macrocell user (or remote station) when the macrocelluser enters the picocell. Note that there is a dramatic drop inE_(b)/N_(t) when the remote station in the macrocell is almost at thesame location as the picocell. This is due to the very strong signalfrom the picocell and the assumption that the picocell and macrocellcannot be made perfectly orthogonal. In FIG. 5, it is assumed that thereis a minimum coupling from the picocell to the macrocell. In theexemplary embodiment, this minimum coupling is given as 0.01. Thus, atleast 1% of the picocells power is non-orthogonal to that of themacrocell. However, if the picocell is non-orthogonal, the remotestation in the macrocell receives a substantial amount of power from thepicocell. FIG. 5 shows that if the remote station is within about 40meters of the picocell, the macrocell has to transmit a considerableamount of power in order to maintain the communication with the remotestation. With an orthogonal picocell, the region where the macrocell hasto transmit a lot of power drops to just a couple of meters. Similarly,there is a substantial range increase for the picocell user by havingthe picocell radiate orthogonally to the macrocell. The example in FIG.5 shows that the range increases by about 50% when the remote station iscloser to the macrocell and increases substantially more in the otherdirection.

FIG. 5 shows the effect along line 20 going through picocell 18.However, if the mobile station is not on line 20, the performance can becalculated. For a given distance from the picocell, the performance willbe bounded between that given for the remote station at the samedistance, but on line 20 and being closer to the macrocell and furtherfrom the macrocell.

The present invention has been described in the context of Walshsequence zero which is reserved for the pilot channel in IS-95 systems.Other Walsh sequences can also be used to generated the pilot Walshsequences of the present invention. The selected Walsh sequence and itscomplementary sequence can be used to generate the pilot Walsh sequencesin the manner described above. In the exemplary embodiment, thecomplementary sequence is derived by inverting each bit in the selectedWalsh sequence. Alternatively, the complementary sequence can be asecond basic Walsh sequence. In summary, other basic Walsh sequences canbe utilized and are within the scope of the present invention.

Although the present invention has be described in the context of a CDMAsystem which conforms to the IS-95 standard, the present invention canbe extended to other communication systems. The pilot Walsh sequencescan be generated from the basic Walsh sequence which, in the exemplaryIS-95 system, is 64 chips in length. Basic Walsh sequences of differentlengths can also be utilized and are within the scope of the presentinvention. Furthermore, any orthogonal sequence or approximateorthogonal sequence can also be used and are within the scope of thepresent invention.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A method of wireless communications, comprising: providing pilotdata; covering said pilot data with a first Walsh sequence to generate afirst pilot signal, wherein the first Walsh sequence comprises a firstbasic Walsh sequence concatenated with a complementary sequence of thefirst basic Walsh sequence; and covering said pilot data with a secondWalsh sequence orthogonal to said first Walsh sequence to generate asecond pilot signal, wherein the second Walsh sequence comprises asecond basic Walsh sequence concatenated with a complementary sequenceof the second basic Walsh sequence: wherein the method is performed by abase station.
 2. The method as in claim 1 further comprising providingsaid second pilot signal for a particularized transmission to aparticular area.
 3. A method of wireless communications, the methodcomprising: receiving a pilot signal; and decovering said pilot signalwith a sequence which includes a basic Walsh sequence concatenated witha complementary sequence of said basic Walsh sequence to recover pilotdata from said pilot signal; wherein the basic Walsh sequence is one ofa plurality of Walsh sequences that are available for covering codechannels, and wherein the method is performed by a remote station. 4.The method as in claim 3 further comprising receiving said pilot signalfrom a particularized transmission directed at a particular area.
 5. Anapparatus for wireless communications, comprising: means for providingpilot data; means for covering said pilot data with a first Walshsequence to generate a first pilot signal, wherein the first Walshsequence comprises a first basic Walsh sequence concatenated with acomplementary sequence of the first basic Walsh sequence; and means forcovering said pilot data with a second Walsh sequence orthogonal to saidfirst Walsh sequence to generate a second pilot signal, wherein thesecond Walsh sequence comprises a second basic Walsh sequenceconcatenated with a complementary sequence of the second basic Walshsequence.
 6. The apparatus as in claim 5 further comprising means forproviding said second pilot signal for a particularized transmission toa particular area.
 7. An apparatus for wireless communications,comprising: means for receiving a pilot signal; and means for decoveringsaid pilot signal with a sequence which includes a basic Walsh sequenceconcatenated with a complementary sequence of said basic Walsh sequenceto recover pilot data from said pilot signal, wherein the basic Walshsequence is one of a plurality of Walsh sequences that are available forcovering code channels.
 8. The apparatus as in claim 7 furthercomprising means for receiving said pilot signal from a particularizedtransmission directed at a particular area.
 9. An apparatus for wirelesscommunications, comprising: a first multiplier configured to cover pilotdata with a first Walsh sequence to generate a first pilot signal,wherein the first Walsh sequence comprises a first basic Walsh sequenceconcatenated with a complementary sequence of the first basic Walshsequence; and a second multiplier configured to cover said pilot datawith a second Walsh sequence orthogonal to said first Walsh sequence togenerate a second pilot signal, wherein the second Walsh sequencecomprises a second basic Walsh sequence concatenated with acomplementary sequence of the second basic Walsh sequence.